1. Field of the Invention
This invention relates to controlled release fertilizers and methods for their preparation. More particularly, this invention relates to a new granular homogeneous metal ammonium phosphate-alkylene urea buffered fertilizer composition, containing slow releasing water insoluble nitrogen of two different types, which provides sustained release of nitrogen, phosphorous, and metal nutrients throughout long periods of time, and a method for its effective preparation. The new method and the composition obtained thereby are the results of the discovery that large portions of alkylene ureas may be homogeneously included in a matrix of metal ammonium phosphate to form hard granules with release characteristics improved over both components, and that the water insoluble alkylene urea compounds and polymers may be formed in-situ during the formation of the water insoluble metal ammonium phosphate matrix, from soluble materials.
2. Description of Related Art
Divalent metals form ammonium phosphates, having the general formula where Me represents the metals, MeNH.sub.4 PO.sub.4.XH.sub.2 O, which are slightly soluble in water and soil solutions, and are non-burning sources of nitrogen, phosphorous and metals. These products which release slowly have been known to the art for a long time and magnesium ammonium phosphate has been effectively used as a speciality fertilizer where nitrogen concentrations were not matters of concern. The low concentrations of nitrogen in magnesium ammonium phosphate and in the other metal ammonium phosphates, have severly limited the practical utilities of these products to uses as sources of metal nutrients. The initial release of N is very low from these compounds.
Alkylene urea compounds and polymers have achieved wide commercial acceptance as nitrogen fertilizers and provide much of the controlled, and slow release, nitrogen currently used. Alkylene urea fertilizer compounds, isobutylene diurea, and crotonylidene diurea are used as slow release fertilizers, which depend upon low water solubilities and slow hydrolysis for their nitrogen releases, while ureaformaldehyde polymers depend upon some hydrolysis, but primarily upon microbial degradation for their nitrogen releases. The alkylene urea fertilizers are used as powders, or as granules, which have little resistance to crushing or attrition, and almost no buffer capacity to resist acidification, as the nitrogen is mineralized by soil microbes to nitrate.
Metal ammonium phosphates have been prepared from aqueous ammonia, phosphoric acid, and divalent metal oxides or hydroxides, or from ammonium phosphates and divalent metal oxides, or hydroxides.
Alkylene urea compounds and polymers are usually prepared by acid catalyzed reaction and or, polymerization of basic aqueous urea and alkyl aldehydes.
G. L. Bridger et al in Ind. Eng. Chem. Process Des. Dev. Vol 10, No. 3, 1962, pages 181-188, taught that divalent metals magnesium, iron, zinc, copper, and manganese form ammonium phosphates which are very slightly soluble and are effective, safe, and long lasting sources of nitrogen, phosphorous, and metals for plant foliar and soil nutrition.
P. H. Peng et al in Ind. Eng. Chem. Process Des. Dev. Vol 18, No. 3, 1979, pages 453-458, taught the continuous production of magnesium ammonium phosphate based fertilizers from dry magnesia, monoammonium phosphate and water, and optionally with potassium chloride and micronutrients added. The reaction was not complete throughout the granule. Ammonia losses were reported to be significant in the process. The granules produced in the conventional pan granulation method were noticeably wet, weak, and crumbled upon leaving the granulator and required several hours aging before they were strong enough for screening without breakage.
My U.S. Pat. No. 5,019,148 taught a stepwise method of preparing homogeneous mineral granules, such as the metal ammonium phosphates, by coreaction of acids and bases to form a chemically transient fluid adhesive which hardened to form a strong granule-binding cement. The transient fluid adhesive was comminuted by mechanical means to form plastic globules which were rolled to form spheroid granules.
H. M. Goertz in U.S. Pat. No. 4,378,238 taught preparation of low molecular weight urea formaldehyde compounds and polymers by the acid catalyzed reaction of aqueous urea and formaldehyde. This product contained methylene urea polymers and compounds of varying chain lengths with the majority consisting of short chain polymers of methylene diurea and dimethylene triurea, and is typical of a large part of the solid ureaformaldehyde products currently marketed in North America.
Alkylene urea compounds and low molecular weight polymers are normally supplied and used as powders or granules. The granules usually have little physical integrity and attrition frequently causes dusting and unreliable rates of nitrogen release.
The prior art has provided slow release alkylene urea fertilizers exhibiting high nitrogen concentrations and water insolubilities and rather low resistances to attrition. None of the prior art alkylene urea fertilizers provide buffering of the mineralized nitrogen.
The prior art also has provided slow release metal ammonium phosphate fertilizers as strong granular buffered nitrogen products, but the nitrogen concentrations are too low for effective use as commercial nitrogen fertilizers and the early nitrogen release is slower than needed for many nitrogen fertilizer uses.
Terms used in the plant nutrient industry and herein are defined as follows:
(1) water insoluble nitrogen=CWIN, defined in Official Methods of Analyses, 15th Edition, 1990, Association of Official Analytical Chemists, Vol 1, page 20, 945.01. PA0 (2) mol ratios relating to metal ammonium phosphates=ratio of elements N, P, Metals. PA0 (3) mol ratios relating to alkylene ureas=molar ratio of urea to aldehyde. PA0 (4) moiety=functional chemical part, element, or functional group.